1. Field
The present disclosure relates to an optical device for communicating optical signals. More specifically, the present disclosure relates to an optical device that operates with reduced tuning energy.
2. Related Art
Wavelength division multiplexing (WDM) is widely used to communicate modulated data at different carrier wavelengths on a common optical waveguide. By using different carrier wavelengths, WDM can effectively overcome optical-fiber congestion, which is a potential problem in optical modules that include parallel optical transceivers with one channel per optical fiber. Moreover, by significantly reducing the number of optical fibers per optical module, WDM can simplify optical modules, thereby reducing their cost and size.
Dense WDM (DWDM) is a variation on WDM that uses a narrow spacing between adjacent wavelengths. This is typically achieved by modulating data directly onto a highly stable optical carrier, and then combining multiple carriers in an optical fiber. DWDM allows a large number of channels to be accommodated within a given wavelength band, and thus offers higher performance.
In DWDM systems, a variety of optical devices are used, such as: optical waveguides, optical modulators, optical multiplexers (such as add filters), optical de-multiplexers (such as drop filters), optical proximity couplers, optical filters, optical switches and optical detectors. While some of these optical devices (such as optical waveguides, optical proximity couplers and optical detectors) are broadband components that are relatively insensitive to ambient temperature changes and fabrication variation, wavelength-selective optical devices (such as resonator-based optical modulators, optical multiplexers, optical filters and optical de-multiplexers) can be very sensitive to these changes and variations. In order to compensate for the corresponding changes in the actual operating wavelengths of these wavelength-selective optical devices (relative to predetermined desired or target operating wavelengths), a given wavelength-selective optical device is typically phase-tuned to a particular carrier wavelength of a given channel.
Thermal tuning is a popular tuning technique to perform this phase tuning because it provides the ability to produce large phase shifts. Existing thermal tuning techniques include direct heating (which is implemented by doping in an optical waveguide) and indirect heating (in which a heater is heated in proximity to the optical waveguide). Typically, the direct-heating technique is more energy-efficient than indirect heating, but it can prevent the optical waveguide from performing additional functions (because of the constraint on the doping density), and it can introduce additional optical losses due to free-carrier absorption (which can degrade the quality factor of an optical resonator).
In principle, optical devices can be made on silicon substrates, because silicon provides many benefits for optical communication. For example, the high index-of-refraction contrast between silicon and silicon dioxide can be used to create sub-micron optical waveguides to confine light with spatial densities that are up to 100× larger than in a single-mode optical fiber. Furthermore, by using silicon-on-insulator (SOI) technology, a silicon optical waveguide can be surrounded by silicon dioxide on all four sides, which facilitates low-loss, on-chip optical waveguides and active devices (such as optical detectors and optical modulators). Silicon-based optical devices can additionally be used to implement a wide variety of optical components for use in WDM communication. These silicon-based optical devices offer numerous advantages, including: miniaturization, low-energy modulation, the ability to integrate with other devices in silicon, and/or the ability to leverage the large, existing silicon manufacturing infrastructure.
Unfortunately, there are problems associated with silicon-based optical devices. A notable problem arises from the high thermal conductivity of silicon. While this helps remove the heat dissipated by electrical circuits, it can make it more difficult to thermally tune a silicon-based optical device. In particular, because the actual operating wavelength of a silicon-based optical device (such as the resonant wavelength of an optical resonator) strongly depends on temperature, the actual operating wavelength is typically tuned using either direct or indirect heating to change the operating temperature of the silicon-based optical device. However, the high thermal conductivity of silicon results in excessive thermal coupling to the surrounding environment. Consequently, thermal tuning of silicon-based optical devices often consumes a disproportionately large amount of energy (typically, 50-100 mW for a phase shift of 180°). This high power consumption can offset the advantages provided by silicon, and makes it more difficult to use silicon-based optical devices to implement optical communication (such as WDM) in computing systems (especially in systems that have multiple instances of the optical devices).
Hence, what is needed is an optical device that can be thermally tuned without the above-described problems.